It is known practice to dye human keratin fibres, and in particular the hair, with dye compositions containing oxidation dye precursors, which are generally known as oxidation bases. These oxidation bases are colourless or weakly coloured compounds, which, when combined with oxidizing products, give rise to coloured compounds by a process of oxidative condensation.
It is also known that the shades obtained with these oxidation bases may be varied by combining them with couplers or coloration modifiers. The variety of molecules used as oxidation bases and couplers allows a wide range of colours to be obtained.
This oxidation dyeing process consists in applying to keratin fibres oxidation bases or a mixture of oxidation bases and of couplers with an oxidizing agent, for example aqueous hydrogen peroxide solution, leaving the agents on the fibres and then rinsing the fibres. The colorations resulting therefrom are permanent, strong and resistant to external agents, especially light, bad weather, washing, perspiration and rubbing. This process, which is generally applied at basic pH, makes it possible simultaneously to dye and lighten the fibre, which is reflected in practice by the possibility of obtaining a final coloration that is lighter than the original colour. In addition, lightening of the fibre has the advantageous effect of generating a unified colour in the case of grey hair, and of bringing out the colour, i.e. making it more visible, in the case of naturally pigmented hair.
It is also known practice to dye human keratin fibres with a direct dye. The process conventionally used in direct dyeing consists in applying to the keratin fibres direct dyes, which are coloured and colouring molecules that have affinity for the fibres, leaving the dyes on the fibres and then rinsing the fibres.
It is known practice, for example, to use nitrobenzene, anthraquinone, nitropyridine, azo, xanthene, acridine, azine or triarylmethane direct dyes.
The colorations resulting therefrom are colorations that are temporary or semi-permanent since the nature of the interactions linking the direct dyes to the keratin fibre, and their desorption from the surface and/or core of the fibre are responsible for their poor dyeing power and their poor fastness with respect to washing or perspiration. These direct dyes are also generally light-sensitive due to the poor resistance of the chromophore with respect to photochemical attack, and lead over time to fading of the coloration of the hair. In addition, their light-sensitivity is dependent on their uniform distribution or their distribution as aggregates in the keratin fibre.
It is known practice to use direct dyes in combination with oxidizing agents. However, the direct dyes are generally sensitive to the action of oxidizing agents such as aqueous hydrogen peroxide solution, and reducing agents such as sodium bisulfite, which makes them generally difficult to use in lightening direct dyeing compositions based on aqueous hydrogen peroxide solution and based on a basifying agent, or in oxidation dye compositions in combination with precursors such as oxidation bases or couplers.
Moreover, it is known practice to use, for the dyeing of keratin fibres, dyes containing a hydrazone unit, in particular in documents EP 1 166 757, EP 1 142 559, EP 1 153 599 and EP 1 155 679. In these documents, the dyes are of a different type from those that are useful in the present invention. Document U.S. Pat. No. 3,238,198 also discloses hydrazone dyes of the same type as those that are useful in the present invention. However, these compounds are not described for dyeing human keratin fibres.
There is a real need to find chromatic direct dyes that allow human keratin fibres to be dyed, under mild conditions, as strongly as oxidation dyes, which are just as light-fast as the latter dyes, and which are also resistant to bad weather, washing and perspiration, and also sufficiently stable in the presence of oxidizing and reducing agents to be able simultaneously to obtain lightening of the fibre or to be combined with oxidation dyes.
In addition, there is a need to provide dyes that show good harmlessness and that show less selectivity compared with standard dyes.